net: dsa: mv88e6xxx: Fix writing to a PHY page.

[ Upstream commit c309b158090d788e96ee597444965cb79b040484 ]

After changing to the needed page, actually write the value to the
register!

Fixes: 09cb7dfd ("net: dsa: mv88e6xxx: describe PHY page and SerDes")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>